But Tillerson, who said the US had been "using large sticks" by way of sanctions to convince North Korea to negotiate, also raised the prospect of a conflict with North Korea, saying his diplomatic efforts would continue "until the first bomb drops."

He said:

"Well, that's who we will have to work with to achieve this diplomatically. What we have to determine now is are we even ready to start? Are they ready to start?

"And if they're not, we'll just keep the pressure campaign underway and we will increase that pressure. And we are doing that every month. There are new sanctions rolled out. The world wants North Korea to change ...

"We're not using a carrot to convince them to talk. We're using large sticks. And that is what they need to understand. This pressure campaign is putting — is having its bite on North Korea, its revenue streams. It's having a bite on its military programs ...

"I'm going to use all the time available to me our diplomatic efforts will continue until that first bomb drops. My job is to never have a reason for the first bomb to drop. And we don't know precisely how much time is left on the clock."

Tillerson's message echoes that of Vice President Mike Pence, who told The Washington Post last week that the US was pursuing a policy of "maximum pressure and engagement at the same time" with Pyongyang.

Members of the Trump administration have contradicted one another on North Korea in the past. Three weeks ago, President Donald Trump used his State of the Union address to called North Korea a "cruel dictatorship" embarking on a "reckless pursuit of nuclear missiles."

Tillerson told "60 Minutes" that North Korea "will tell me" when they want to talk, because "we receive messages from them."

He added that North Korea's threats to nuke the US mainland did "make us nervous" but that "it also stiffens our resolve."

The US is said to be considering a "bloody nose" strike on North Korea, in which it would respond to the next North Korean missile or nuclear test with a limited military strike designed to embarrass the country. State Department officials and senators have rejected those claims.